Lactoperoxidase (LP), hydrogen peroxide (H2O2) and thiocyanate ion (SCN-) form a bacteriostatic system in human saliva. The oral "lactic-acid bacteria" release H2O2 as a by-product of carbohydrate metabolism. Stimulated leukocytes in the salivary glands, oral mucosa, and saliva may also produce H2O2 and secrete peroxidase enzymes. Peroxidases catalyze the oxidation of SCN- by H2O2 to yield hypothiocyanite ion (OSCN-), which is in acid-base equilibrium with hypothiocyanous acid (HOSCN). HOSCN oxidizes essential sulfhydryl groups of bacterial enzymes and transport systems, resulting in inhibition of metabolism and growth. Certain oral bacteria such as S. mutans have a limited resistance to inhibition but are inhibited at acid pH. All bacteria can recover and resume metabolism and growth when OSCN- is removed by dilution or reduction. The concentrations of OSCN- found in human saliva do not appear toxic to human cells at neutral pH. The first aim is to evaluate the toxicity of the LP system relative to that of H2O2 to microorganisms (S. mutans, E. coli, S. aureus, C. albicans) and human fibroblasts in vitro, so as to determine whether the primary role of the LP system is to produce a microbistatic agent or to protect host tissues against H2O2 toxicity. The second aims is determine the role of leukocyte peroxidase enzymes myeloperoxidase and eosinophil peroxidase in the H2O2-dependent antimicrobial activity of saliva. The amount and activity of the leukocyte enzymes will be measured in saliva, and the effect of adding more of the enzymes on antibacterial activity will be determined. The third aim is to investigate the role of nitrite and reactive nitrogen-oxides in oral microbial ecology and examine their interactions with the LP system. The hypothesis is that the nitrite produced in saliva becomes microbistatic at low pH due to the formation of nitric oxide (NO ) and may provide a selective advantage to oral streptococci by more effectively inhibiting the energy metabolism of other microorganisms. Nitrite may also potentiate the toxicity of the LP system at low pH through a cooperative attack of HOSCN and NO on microbial sulfhydryl groups. The goal of the project is to achieve a greater understanding of the role of oxidizing agents in oral health and disease.